Why is there variation in plant growth?

Each table group planted cress seeds. The instructions given as to how many seeds and how much water was left open to the children to decide. Children have been investigating why some of the group's cress seeds have successfully grown and why some of them have not. 

"I think that group 1 and 2 haven't watered theirs enough. Also maybe they are not getting enough light from the sun"

"Table five's cress is growing like a forest. We need to find out exactly how many seeds, how much water they used and where they placed their plant as this group has had successful growth"